1. Field of Invention
This invention relates to a method of coating the ejector of an inkjet printhead and to the ejector surfaces so coated.
2. Description of Related Art
Acoustic inkjet printing processes are known. See, for example, U.S. Pat. No. 6,255,383 to Hanzlik, incorporated by reference herein in its entirety. As described therein, an acoustic beam exerts a radiation pressure against objects upon which it impinges. Thus, when an acoustic beam impinges on a free surface (i.e., liquid/air interface) of a pool of liquid from beneath, the radiation pressure which it exerts against the surface of the pool may reach a sufficiently high level to release individual droplets of liquid from the pool, despite the restraining force of surface tension. Focusing the beam on or near the surface of the pool intensifies the radiation pressure it exerts for a given amount of input power. These principles have been applied to prior ink jet and acoustic printing proposals. For example, K. A. Krause, xe2x80x9cFocusing Ink Jet Head,xe2x80x9d IBM Technical Disclosure Bulletin, Vol. 16, No. 4, September 1973, pp. 1168-1170, the disclosure of which is totally incorporated herein by reference, describes an ink jet in which an acoustic beam emanating from a concave surface and confined by a conical aperture is used to propel ink droplets out through a small ejection orifice.
Acoustic ink printers typically comprise one or more acoustic radiators for illuminating the free surface of a pool of liquid ink with respective acoustic beams. Each of these beams usually is brought to focus at or near the surface of the reservoir (i.e., the liquid/air interface). Furthermore, printing conventionally is performed by independently modulating the excitation of the acoustic radiators in accordance with the input data samples for the image that is to be printed. This modulation enables the radiation pressure which each of the beams exerts against the free ink surface to make brief, controlled excursions to a sufficiently high pressure level for overcoming the restraining force of surface tension. That, in turn, causes individual droplets of ink to be ejected from the free ink surface on demand at an adequate velocity to cause them to deposit in an image configuration on a nearby recording medium. The acoustic beam may be intensity modulated or focused/defocused to control the ejection timing, or an external source may be used to extract droplets from the acoustically excited liquid on the surface of the pool on demand. Regardless of the timing mechanism employed, the size of the ejected droplets is determined by the waist diameter of the focused acoustic beam.
Acoustic ink printing is attractive because it does not require the nozzles or the small ejection orifices which have caused many of the reliability and pixel placement accuracy problems that conventional drop on demand and continuous stream ink jet printers have suffered from. The size of the ejection orifice is a critical design parameter of an ink jet because it determines the size of the droplets of ink that the jet ejects. As a result, the size of the ejection orifice cannot be increased, without sacrificing resolution. Acoustic printing has increased intrinsic reliability because there are no nozzles to clog. As will be appreciated, the elimination of the clogged nozzle failure mode is especially relevant to the reliability of large arrays of ink ejectors, such as page width arrays comprising several thousand separate ejectors. Furthermore, small ejection orifices are avoided, so acoustic printing can be performed with a greater variety of inks than conventional ink jet printing, including inks having higher viscosities and inks containing pigments and other particulate components. It has been found that acoustic ink printers embodying printheads comprising acoustically illuminated spherical focusing lenses can print precisely positioned pixels (i.e., picture elements) at resolutions which are sufficient for high quality printing of relatively complex images.
It has also has been discovered that the size of the individual pixels printed by such a printer can be varied over a significant range during operation, thereby accommodating, for example, the printing of variably shaded images. Furthermore, the known droplet ejector technology can be adapted to a variety of printhead configurations, including (1) single ejector embodiments for raster scan printing, (2) matrix configured ejector arrays for matrix printing, and (3) several different types of pagewidth ejector arrays, ranging from single row, sparse arrays for hybrid forms of parallel/serial printing to multiple row staggered arrays with individual ejectors for each of the pixel positions or addresses within a pagewidth image field (i.e., single ejector/pixel/line) for ordinary line printing.
Inks suitable for acoustic ink jet printing typically are liquid at ambient temperatures (i.e., about 25xc2x0 C.), but in other embodiments the ink is in a solid state at ambient temperatures and provision is made for liquefying the ink by heating or any other suitable method prior to introduction of the ink into the printhead. Images of two or more colors can be generated by several methods, including by processes wherein a single printhead launches acoustic waves into pools of different colored inks. Further information regarding acoustic inkjet printing apparatus and processes is disclosed in, for example, U.S. Pat. Nos. 4,308,547, 4,697,195, 5,028,937, 5,041,849, 4,751,529, 4,751,530, 4,751,534, 4,801,953, and U.S. Pat. No. 4,797,693, the disclosures of each of which are totally incorporated herein by reference.
A major source of ink jet misdirection is associated with improper wetting of the surface of the acoustic ink jet printhead. One factor which adversely affects directional accuracy is the interaction of ink accumulating on the surface of the printhead with the ejected ink droplets. Ink may accumulate on the printhead surface after extended expelling of the droplets of ink from the printhead. When the accumulating ink on the printhead surface makes contact with ink to be expelled, a resulting imbalance of the forces acts on the ejecting ink, which in turn leads to misdirection of the ejected ink. This wetting phenomenon becomes more troublesome after extensive use as the array face oxidizes or becomes covered with a dried ink film, leading to a gradual deterioration of the image quality that the printhead is capable of generating. To retain good ink jet directionality, it is desirable to reduce the wetting of the surface of the printhead.
Thus, the construction and operation of an acoustic ink jet printhead requires that a hydrophobic coating be coated on the inside surfaces of the inkjet printhead such that inks in a solvent do not wet the surfaces of the construction. The ejector surfaces of the printhead must be uniformly coated with the hydrophobic coating material. A uniform thickness of the coating is preferred to provide predictable, accurate printing.
In U.S. Pat. No. 5,451,992 to Shimomura et al., an ink jet head is described that is subjected to a liquid repellency treatment. The liquid repellency treatment is applied to at least a peripheral portion of a discharge port of the ink jet head. A mixture of a fluorine-containing high polymer compound and a compound having fluorine substituted hydrocarbon group and a silazane group, alkoxysilane group or halogenized silane group is employed as a liquid repellent agent. Shimomura describes that an absorbing member is immersed in the liquid repellency agent. The absorbing member is then applied to the discharge port of the inkjet head, thereby coating the liquid repellency treating agent on the discharge port. TEFLON(copyright) AF is described as a possible fluorine-containing high polymer compound.
In U.S. Pat. No. 3,946,398 to Kyser et al., a recording apparatus and method is disclosed which includes feeding a writing fluid source to a drop projection means which ejects a series of droplets of writing fluid from a nozzle in a discontinuous stream with sufficient energy to traverse a substantially straight trajectory to a recording medium. Kyser describes that TEFLON(copyright) may be used to coat the ejection surface of the apparatus to maintain a contact angle of greater than 90xc2x0 between the writing fluid and the ejection surface. Kyser does not describe how the TEFLON(copyright) coating is applied to the ejection surface.
In European Patent No. 0 359 365 to Anderson et al., a method of modifying an ink jet head comprising applying a layer of a coating material to the ink jet head to maintain a contact angle of at least about 50xc2x0 at an operating temperature of at least about 70xc2x0 C. is described. The coating material may contain fluorocarbon polymers such as TEFLON(copyright) PTFE (polytetrafluoroethylene) and TEFLON(copyright) PFA (polyperfluoroalkoxybutadiene). Methods to apply the coating material include dip, spray, spin coating, plasma polymerization and the use of electroless nickel. Anderson does not describe drawing the coating material through the interior of the inkjet head.
In U.S. Pat. No. 5,212,496 to Badesha et al., an ink jet recording head comprising a plurality of channels is described. The channels are capable of being filled with ink from an ink supply and terminate in nozzles on one surface of the printhead. The surface is coated with a polyimide-siloxane block copolymer. The coating material can be applied to the surface of the printhead by dissolving the polyimide-siloxane copolymer in a suitable solvent and applying the solution to the surface by spray coating, spin coating, contact coating by use of brushes, fine bristled brushes, rubber rollers, cotton, cloth or foam rubber and applicators, or hand coating with a swab such as a Q-TIP(copyright) and allowing the solution to evaporate. Examples of suitable solvents include dichloromethane, methyl ethyl ketone, tetrahydrofuran, and N-methylpyrrolidone.
In one embodiment, it is described to use pressurized gas to prevent the interior channel walls from becoming coated with the solution. It is further described that if ink-repellent material coats the walls of the channels, then the proper refill of each channel after firing of a droplet is inhibited resulting in misdirection or drop size variability. In one embodiment described therein, the ink-repellent coating is applied to the printhead array face while blowing high velocity filtered gas through the array. In this embodiment, the strong gas stream inhibits the ink-repellent material from entering the channels and coating the walls. This technique is highly effective for ensuring that only the front face receives a coating of repellent and not the channel walls, i.e., the inside surfaces of the printhead.
Thus, Badesha describes methods to avoid coating the interior channels with the solution, and in one embodiment uses pressurized air to prevent the solution from entering the channels. Badesha does not describe using the pressurized air to draw a coating solution through the printhead.
What is desired is a method of coating the ejectors of ink jet printheads with a uniform coating of ink-phobic material. Also desired is a method of coating the inside ejector surfaces of ink jet printheads with a uniform coating of ink-phobic material.
It is an object of the present invention to provide a method of coating the ejector and/or ejector surfaces of an ink jet printhead.
It is another object of the present invention to provide a method of coating the inside surfaces of the ejector or inside ejector surfaces of an acoustic ink jet printhead.
It is a further object of the present invention to provide uniform coatings of an ink-phobic coating.
It is still a further object of the present invention to provide a method of coating ink jet printheads that is economical and efficient.
It is a still further object of the present invention to provide a coating for the ejector and/or ejector surfaces of ink jet printheads exhibiting resistance to corrosive inks.
It is a still further object of the present invention to provide a printhead having an ejector coated with TEFLON(copyright) AF fluoropolymers.
These and other objects of the present invention are achieved by coating the ejector surfaces of an ink jet printhead with an ink-phobic coating and drawing the ink-phobic material through the inside of the ejector of an ink jet printhead.